The purpose of the proposed research project will be to use stacked regions of photosynthetic membranes of plants, algae and bacteria as model systems for studying molecular mechanisms of membrane-membrane interactions. During the past year we have positively identified the chlorophyll a/b light harvesting complex (chl a/b LHC) as the adhesion factor of chloroplast membranes of higher plants and of green algae. We now intend to identify and characterize the component(s) of the complex that are responsible for its adhesive properties using a combination of biochemical, electron microscopical and immunological procedures, and by taking advantage of selected mutants. Attempts will be made to isolate and purify photosystem II - chl a/b LHC particles from chloroplast membranes, and to test the adhesion properties of these particles in reconstituted liposomes. We will also investigate the molecular basis of regulation of thylakoid membrane adhesion that occurs during the cell cycle of Chlamydomonas reinhardtii. Finally, we will examine the structural and molecular basis for adhesion between photosynthetic membranes of Rhodopseudomonas palustris and Rps. viridis, so as to be able to compare a related type of membrane-membrane interaction in a plant and a bacterial system.